Orthopaedic surgeons have been applying the principles of tissue engineering for years, while transplanting and shifting matrices within patients to promote regenerative potential. The advent of new technology now offers even greater promise and brings unbridled enthusiasm that full regenerative potential of tissue and whole organ systems can be achieved in the near future. While soft tissue repair can be managed by achieving scar tissue replacement, such outcome in most orthopaedic applications and indications would be insufficient. Bone requires a tissue-specific composition to be attendant to function for skeletal support. The forming of collagenous material alone, even if vascularized, will fail to meet the biophysical demands of repetitive skeletal loading and be inadequate.
Implicit in the goals of repairing bone are to achieve restitution of space, mechanical solidarity, and functional continuity. Often the biological signals do not provide sufficient stimulus to attain a full repair. Orthopaedic interventions to alleviate fracture non-union, pseudarthrosis, and scoliosis; bone defects due to congenital or developmental anomalies, infection, malignancy, or trauma often require bone grafting to augment the process of bone healing. The therapeutic goal of graft material is to omit compliance features such as strain tolerance, reduced stiffness, and attenuated strength, and instead promote primary, or membranous-type bone formation within the physical approximation of graft material. In order to achieve this, three basic components are required: osteoprogenitor cells, osteoinductive factors, and an osteoconductive matrix or scaffold.
Autologous cancellous bone remains to date the most effective graft material, where osteoinductivity, osteoconductivity, and a rich source of cells endow the material with not only biological activity but a degree of immunologic transparency as well. Because of complications and shortcomings associated with autogenous grafting that include limited quantity, donor-site morbidity, and more recently cost consideration (St. John T. A., et al., Am. Journal of Orthopedics; 32:18-23, 2003), numerous alternative graft materials have been developed for orthopaedic applications. All references as cited herein are incorporated in their entireties for the purposes of the present invention.
Allograft bone is used extensively as a material bridge to osseo-integration, acknowledged as a substitute for the general short supply of autograft. Primary uses to date have been in spine, although both trauma and plastics account for a growing market. The market offers several options, the most valuable being the machined components for surgical implantation. Over 675,000 procedures set the demand for allograft annually, with a projected market growth set at 19%.
Available grafting substitutes include cancellous and cortical allograft bone ceramics such as sintered coralline matrices, hydroxyapatite and tri-calcium phosphate, demineralised bone matrix, bone marrow, composite polymer grafts, and recently recombinant cytokines with collagen carriers. Complications include availability, cost, variable bioabsorption, brittleness, immune stimulation, and regulatory hurdles.
The shape of the biomaterial template is critical to the success of manufacturing. A central tenet of biomineralization is that nucleation, growth, morphology and aggregation of the inorganic crystals of bone are regulated by organized assemblies of organic macromolecules. The close spatial relationship of hydroxyapatite crystals with Type I collagen fibrils in the early stage of bone mineralization is a relevant example. It is equally evident that combining hydroxyapatite with protein does not render the macroscopic form of bone nor impart its characteristic properties. Unlike fabricated materials that can be developed from components with predictable properties (Olson G. B., Science; 277: 1237-1242, 1997), biological systems control desired properties by utilizing an intrinsic rationale that discriminates essential from non-essential factors. Living organisms avoid the geometric frustration of randomness by segregating structures that resonate function.
Future envisaged bio-engineering strategies will combine several favourable properties of the current items in an effort to achieve hybrid materials that support tissue differentiation without shielding capacity for integrated modelling.
Ideally, new materials will provide tissue compatibility and minimize patient morbidity.
Although bone can appear de novo, it more often develops from accretion on a scaffold of matrix that contains appropriate vascular and compositional arrangement. As such, both 2-dimensional and 3-dimensional patterns have been shown to enhance osteoconductivity (Liao H., et al., Biomaterials 24: 649-54, 2003). Bone has significantly more matrix than cells, and cell regulation through anchorage dependent mechanisms is an established premise (Clover J. and Dodds R. A., J. Cell Sci; 103: 267-271, 1994; Ingber D. E., Int. Rev. Cytol.; 150: 173-224, 1994; Meazzini M C et al., J. Orthop. Res.; 16: 170-180, 1998). Compensatory mechanisms for changing sensitivity to mechanical stimulation have been shown to undergo adaptive or kinetic regulation, likely tied, directly to osteoblast attachment to immobilized molecules in the extracellular matrix (ECM). ECM molecules promote cell spreading by resisting cell tension, thereby promoting structural rearrangements within the cytoskeleton (Ingber D. E., Annu. Rev. Physiol.; 59: 575-599, 1997). Several lines of evidence suggest that tension or mechanical stretch exerts a direct positive effect on bone cells and bone cell differentiation through: 1.) activation of phospholipase A2, 2.) release of arachidonic acid; 3.) increased prostaglandin E synthesis, 4.) augmented cyclic adenosine monophosphate (cAMP) production; and 5.) and expression of the bone associated transcription factor CBFA-1 (Bindermann I., et al., Calcif Tissue Int.; 42: 261-266, 1988; Somjen D., et al., Biocim. Biophys. Acta; 627: 91-100, 1980; Yeh C. K. and Rodan G. A., Calcif Tissue Int.; 36: S82-85, 1984; Nikolovski J., et al., FASEB J.; 17: 455-7, 2003). It has long been recognized that a sustained increase in the cellular level of cAMP constitutes a growth-promoting signal (Rozengurt E., et al., J. Cell Biol.; 78: 4392-4396, 1981), and that prostaglandins directly affect a change in cell shape and increase intercellular gap junctions (Shen V., et al., J. Bone Miner Res.; 1: 2443-249, 1986). Without a capacity for attachment and spreading, cells undergo apoptosis, or programmed cell death (Chen C. S., et al., Science; 276: 1425-1428, 1997; Edmondson A. C., Bosten, 1987).
Bone withstands compressive loading by efficient distribution of internal tensile forces. Bone cells do however adhere to structures that can resist compression in order to spread, engaging osteoblast attachment, mineralization, and bone matrix organization as linked processes. Even though deformation at the tissue level might be evaluated as an ability to resist compression, force along individual trabeculae reflects an ordinate of new tension. Under normal cycles of development, increased mass conveys a progressive stimulus of tension to cells, gravity imposing a unidirectional vector to terrestrial life.
A sudden reduction in gravity imposes serious consequence to the skeleton. As shown by studies of astronauts, marked skeletal changes in the weight-bearing skeleton including a reduction in both cortical and trabecular bone formation (Jee W. S. S., et al., Am. J. Physiol.; 244: R310-R314, 1983), alteration in mineralization patterns (Zerath E., et al., J. Appl. Physiol.; 81: 194-200, 1996), and disorganization of collagen and non-collagenous protein metabolism (Backup P. K., et al., Am. J. Physiol.; 266: E567-E573, 1994) have been associated with microgravity. Each month of spaceflight results in a 1-2% reduction of bone mineral density that has been linked to down-regulated PTH and 1,25-dihydroxyvitamin D3 production (Holick M F, Bone; 22: 105S-111S, 1998). Indices from cosmonauts aboard Euromir 95 account bone atrophy to both a reduction in bone formation and increased resorption. PTH decreased (48%), as did bone alkaline phosphatase, osteocalcin, and type-I collagen propeptide. At the same time bound and free deoxypyridinoline and pro-collagen telopeptide increased (Caillot-Augusseas A., Lafage-Proust M H et al., Clin. Chem.; 44: 578-585, 1998). The chords of information establish a role for microgravity in uncoupling bone formation and enhancing resorption.
If exposure to microgravity demonstrates physiologic responses that mirror a reduction in trabecular tension, then would reciprocity of function be expected in bone that is modelled under microgravity and then exposed to normal gravitational force? Prolonged weightlessness, as experienced in space flight effectively unloads the skeleton, relaxing tension on the trabeculae. In this manner, osteoblast physiology preferably is altered due to attachment perturbations. Conversely, a bioscaffold modelled in the form of tissue that has developed under microgravity, will experience an enhanced tensile loading sensation on individual trabeculae.
In view of the above it is therefore the object of the present invention is to provide a scaffold that preferably is not only structurally enhancing but at the same time inductively optimum for bone formation. This graft should be designed to stimulate cell differentiation and bone regeneration, and to be utilized as an orthotopic alternative to tissue transplantation.